Norman,

I would schedule a down time slot of about oooooh 5 seconds (depending no your nearest layer 3 device), attach my console cable to the back of the primary - configure on your inside interface the command:-

ip address x.x.x.x y.y.y.y standby z.z.z.z

(also depending on your failover config/requirements)

As long as your/if you are using it:-

failover link failover <>

failover interface ip failover <> <> standby <>

does not change - can't see an issue!

Oooo and wr mem - to sync the configs of course!

HTH>